Changes In Secular Variation Research Articles

Geomagnetic jerks observed in geomagnetic observatory data over southern Africa between 2017 and 2023

Abstract Geomagnetic jerks are jumps observed in the second derivative of the main magnetic field that occur on annual to decadal timescales. Understanding these jerks is crucial as they provide valuable insights into the complex dynamics of the Earth’s outer liquid core. In this study, we investigate the occurrence of geomagnetic jerks in geomagnetic observatory data collected at southern African magnetic observatories, Hermanus (HER), Tsumeb (TSU), Hartebeesthoek (HBK) and Keetmanshoop (KMH) between 2017 and 2023. The observatory data was processed and analysed by retaining quiet night-time data recorded during quiet geomagnetic activities with the help of planetary K (Kp), Disturbance storm time (Dst) and ring current (RC) indices. Results confirm the occurrence of 2019 to 2020 geomagnetic jerk in the region, and identify the recent 2021 jerk detected with V-shaped secular variation changes in X and Z components at all four observatories. The highest estimated 2021 jerk secular acceleration amplitudes in X and Z components were found at HBK, 12.7 nT/year2 and 19.1 nT/year2, respectively. Notably, the global CHAOS-7 model aptly identifies this 2021 jerk in the Z component at all magnetic observatories in the region.

Archaeointensity record of weak field recurrence in Japan: new data from Late Yayoi and Kofun ceramic artifacts

Summary We present new absolute archaeointensity data from six archaeological sites situated in the Okayama Prefecture, Japan. The materials studied are well-dated fragments from pottery, ceramic coffins and haniwa artifacts. Their ages range from 160 AD to 675 AD, covering the Late Yayoi and Kofun periods. Rock magnetic experiments suggest the presence of magnetite and/or Ti-magnetite as the main carrier of the remanence, with a possible minor contribution of higher coercivity minerals. After thermal demagnetization experiments, the most magnetically stable samples were selected for archaeointensity analysis performed following the double-heating method proposed by Thellier and modified by Coe. Partial thermoremanent magnetization (pTRM) checks and pTRM tail-checks were performed for monitoring possible chemical alterations during heating. All measurements were corrected for both anisotropy and cooling-rate effects. Successful archaeointensity determinations, following rigorous selection criteria, were obtained for samples from all the investigated archaeological sites. Compared with literature data from Japan, the new high-quality data show significantly lower intensity values. They also reveal possible fast secular variation changes during the Late Yayoi period and very weak geomagnetic intensity field around 630 AD. Such values offer evidence of a possible recurrence of weak intensity field in East Asia, suggesting an ancient recurrence of the West Pacific Anomaly. The new data might change the archaeomagnetic field models interpretations in the area, even though more data are still necessary to better understand the secular variation in Japan and the temporal evolution of the geomagnetic field's behavior in East Asia.

A regional geomagnetic field model over Southern Africa derived with harmonic splines from Swarm satellite and ground-based data recorded between 2014 and 2019

A regional harmonic spline geomagnetic main field model, Southern Africa Core Field Model (SACFM-3), is derived from Swarm satellite and ground-based data for the southern African region, in the eastern part of the South Atlantic Anomaly (SAA) where the field intensity continues to decrease. Using SACFM-3 and the global CHAOS-6-×9 model, a detailed study was conducted to shed light on the high spatial and temporal geomagnetic field variations over Southern Africa between 2014 and 2019. The results show a steady decrease of the radial component Z in almost the entire region. In 2019, its rate of decrease in the western part of the region has reached high values, 76 nT/year and 78 nT/year at Tsumeb and Keetmanshoop magnetic observatories, respectively. For some areas in the western part of the region the radial component Z and field intensity F have decreased in strength, from 1.0 to 1.3% and from 0.9 to 1.2%, respectively, between the epochs 2014.5 and 2019.5. There is a noticeable decrease of the field intensity from the south-western coast of South Africa expanding towards the north and eastern regions. The results show that the SAA area is continuing to grow in the region. Abrupt changes in the linear secular variation in 2016 and 2017 are confirmed in the region using ground-based data, and the X component shows an abrupt change in the secular variation in 2018 at four magnetic observatories (Hermanus, Hartebeesthoek, Tsumeb and Keetmanshoop) that needs further investigation. The regional model SACFM-3 reflects to some extent these fast core field variations in the Z component at Hermanus, Hartebeesthoek and Keetmanshoop observatories.Graphical

Signs of a new geomagnetic jerk between 2019 and 2020 from Swarm and observatory data

Following the observed pattern of a new geomagnetic jerk every 3–4 years, certain predictions suggested that a new event should occur around 2020 after the one observed around 2017.5. In this work, we explore this scenario by analysing the secular variation of the East geomagnetic field component in both ground and satellite geomagnetic data. At ground, we use the available data from 2015 to 2021 in 10 observatories worldwide distributed. This analysis shows the occurrence of the mentioned jerk in mid-2017 at observatories located in the Pacific region, but also reveals a new jerk between mid-2019 and early 2020 with a clear global character. Swarm satellite data also corroborate these findings by means of the secular variation estimated using virtual observatories at 440 km altitude. In addition, a general view using the most recent CHAOS geomagnetic model confirms the global character of the 2020-jerk with V-shaped secular variation changes in meridional sectors covering the Eastern Pacific, America, Asia and the Indian Ocean; and Λ-shapes in Europe, Africa and Western Pacific. The radial geomagnetic field at the core–mantle boundary is investigated as the origin of the new jerk. Results show that the global-average secular acceleration of the radial field exhibits a new pulse at mid-2018, establishing the starting epoch of the 2020-jerk.

Geomagnetic secular variation changes in Southern Africa during the SWARM period 2013 - 2018

Geomagnetic field observations from 4 magnetic observatories located in Southern Africa located at Hermanus (HER), Hartebeesthoek (HBK), Keetmanshoop (KMH) and Tsumeb (TSU) have been analysed with the main purpose to identify abrupt secular variation changes on time scales of less than 1 year. Removal of an annual variation resulting from large-scale magnetospheric and ionospheric currents by means of 12-month differences of the respective observatory monthly mean of northward component X, eastward component Y and vertical component Z, revealed clear evidence of several geomagnetic secular variation changes that took place in this region during the period between 2013 and 2018. The geomagnetic field model CHAOS6-x7, based exclusively on SWARM satellite and magnetic observatory data during this period, has been used to determine secular acceleration patterns across Southern Africa. The results obtained revealed that the observed secular variation changes took place over a range of strengths in the respective X, Y and Z components at every magnetic observatory. In addition, the respective observatories in the region also exhibited strong individual characteristics. The findings in this investigation show once again that the southern African region is characterised by unpredictable abrupt geomagnetic secular variation changes that have the potential to render linear prediction models less accurate.

Statistical evaluation of global geomagnetic field models over Southern Africa during 2015

Global geomagnetic field models using spherical harmonic basis functions are important in space physics research, space weather and applications like navigation and mineral resources exploration. These models are based on various geomagnetic field data sets ranging from Earth surface magnetic observatory measurements to low-Earth orbit satellites equipped with highly sensitive and accurate magnetometers. Although these field models are derived by fitting harmonic functions to data distributed across the Earth, they are applied on regional scales within fixed boundaries in many instances and one can therefore question how well do these models perform on restricted areas. Three recently published global geomagnetic field models, IGRF-12, CHAOS-6 and POMME-10, have been statistically evaluated over Southern Africa using repeat station data as well as measurements from 4 INTERMAGNET observatories located at Hermanus and Hartebeesthoek in South Africa as well as Tsumeb and Keetmanshoop in Namibia for 2015. Apart from the observatory data, the field survey repeat station data do not form part of the data set on which these global field models are based and therefore can be regarded as an independent test of these field models over an area like Southern Africa which is well known for its rapid change of the geomagnetic field. Results obtained in this investigation for both main field and secular variation models clearly showed the importance of timely ground-based geomagnetic field observations in the derivation of accurate field models, particularly in regions characterised by rapid and unpredictable secular variation changes.

Valuation of induction current contribution to the secular variation of geomagnetic field (according to the data of Ukrainian geomagnetic observatories)

Induction current contribution to the secular variation of geomagnetic field in accordance with the data of Ukrainian geomagnetic observatories has been investigated. The dynamics of the monthly changes of the secular variations depending on the values of the induction currents has been shown. Correlation between changes of secular variations and solar activity has been found. Changes of solar activity outstrip secular variation changes.

SOUTHERN AFRICAN GEOMAGNETIC SECULAR VARIATION FROM 2005 TO 2009

The southern African region is in close proximity to the South Atlantic Anomaly, a region between Africa and South America where the geomagnetic field is significantly weaker than at other comparable latitudes and is decreasing strongly. Between 2005 and 2009 very dense annual magnetic repeat station surveys were conducted in southern Africa within the framework of COMPASS (Comprehensive Magnetic Processes under the African Southern Sub-continent). This project aims at studying the regional geomagnetic field and particularly its evolutionary behaviour as part of the Inkaba yeAfrica cooperative project between Germany and South-Africa. We have modelled the magnetic field and its secular variation by means of two different techniques, one based on surface polynomials, the other on harmonic splines. Both approaches succeed in describing the characteristic time variations of the magnetic field components in this area. The rapid changes observed in the declination and vertical component during the period of investigation and revealed by the modelling results are of particular interest. Secular variation changes observed in the time-series of the Hermanus and the recently established Keetmanshoop magnetic observatories reveal the occurrence of a geomagnetic jerk in 2007.

SIGNATURE OF THE 2007 GEOMAGNETIC JERK AT THE HERMANUS MAGNETIC OBSERVATORY, SOUTH AFRICA

Quiet-time mean monthly values from the INTERMAGNET observatory at Hermanus (HER) in South Africa were used to study the changes in secular variation during the period between 2005 and 2009. After removing an annual variation resulting from magnetospheric and ionospheric currents by means of a 12-month running means applied to the respective observatory first differences of the X, Y, and Z components, clear evidence was revealed of a strong geomagnetic jerk that occurred during 2007 in this area. The GRIMM-2X model also provided evidence of the occurrence of this jerk in 2007. Of particular interest is that GRIMM-2X predicts the turning points in all the secular variation trends to occur much earlier than revealed by the observatory data. We also observed that the power of this jerk, determined as the difference in slope of the secular variation before and after the jerk, is several times stronger than the global jerk of 1982/3.

Four decades of European geomagnetic secular variation and acceleration

Geomagnetic secular variation, the generally slow, continuous change in the core magnetic field, is characterized by occasional rapid variations known as geomagnetic jerks. Recent studies on magnetic data obtained by satellites with a good global coverage suggest that more rapid and smaller scale features than previously thought occur in the field change. We have taken advantage of the comparatively high density of geomagnetic observatories in Europe and have derived a regional model for the detailed study of secular variation and acceleration over the past four decades from 1960 to 2001 by means of improved and regularized spherical cap harmonic analysis. We show the improvements to our regional model over a global model. All the known jerks are seen in our model, but further times with rapid changes in secular variation exist. Moreover, times of zero acceleration in general do not occur simultaneously in all magnetic field components, although this nearly is the case in 1969.6 and 1982.2. Secular variation and acceleration show very dynamic patterns indicating rapid and complex causal processes in the Earth’s fluid core.

A historical declination curve for Munich from different data sources

Long series of geomagnetic field changes are important for studying processes in the Earth’s core. We have compiled 635 historical declination values for southern Germany and surrounding areas. Indirect estimations, including the oldest ones from back in the 15th century, come from 185 sundials and compasses with declination information, and 15 historical maps. Measurements carried out by monks in the time interval 1668–1854 amount to 122 annual mean values and data related to the orientation of mine shafts contributes by 313 annual values for several locations. All these data can be useful to improve historical geomagnetic field models. Previously compiled German church orientations, however, are shown to be no reliable sources of the past declination. The compiled new declination curve for Munich shows general agreement with previously published curves for London and Paris and allows to detect geomagnetic jerks with a temporal uncertainty of ± 10 years. More or less regular impulses, on a decadal time-scale ranging from 30 to 60 years, are identified for most of the time interval 1400–2000, but the century from about 1760 to 1860 seems to be devoid of sudden secular variation changes.

Longitudinally confined geomagnetic reversal paths from non-dipolar transition fields

IT has long been thought that conditions at the boundary between the core and mantle influence the Earth's magnetic field, but the supporting evidence is rather indirect1–3. Recent palaeomagnetic results, suggesting that there are persistent preferred longitudinal paths for the virtual geomagnetic pole (VGP) during reversals4, would provide the first direct evidence of the solid mantle's influence on the core, although their statistical significance has been disputed5,6. The results are potentially exciting because the preferred paths lie close to the Pacific rim, where the present geomagnetic secular variation changes character2,7. Here we present a simple model, based on an extension of a previous theory8, that produces reversals with VGP paths confined within relatively narrow longitude bands despite the transition field having a substantially non-dipolar structure. Thus, although longitude bias of the VGP paths is definitive evidence for core-mantle interaction, simple VGP paths are not evidence of near-dipolar transition fields.

More on the alleged 1970 geomagnetic jerk

French and United Kingdom workers have published reports describing a sudden change in the secular acceleration, called an impulse or a jerk, which took place around 1970. They claim that this change took place in a period of a year or two and that the sources of the alleged jerk are internal. An earlier paper by this author questioned their method of analysis pointing out that their method of piecemeal fitting of parabolas to the data will always create a discontinuity in the secular acceleration where the parabolas join and that the place where the parabolas join is an a priori assumption and not a result of the analysis. This paper gives a very brief summary of this first paper and then adds additional reasons for questioning the allegation that there was a worldwide sudden jerk in the magnetic field of internal origin around 1970. These new reasons are based largely on new field models which give cubic approximations of the field right through the 1970 timeframe and therefore have no discontinuities in the second derivative (jerk) around 1970. Some recent Japanese work shows several sudden changes in the secular variation pattern which cover limited areas and do not seem to be closely related to each other or to the irregularity noted in the European area near 1970. The secular variation picture which seems to be emerging is one with many local or limited-regional secular variation changes which appear to be almost unrelated to each other in time or space. A worldwide spherical harmonic model including coefficients up to degree 13 could never properly depict such a situation.

Geomagnetic Secular Variation and its Changes, 1942.5 to 1962.5

A uniform set of geomagnetic data from observatories has been analysed to determine secular variation of the geomagnetic field for the interval 1942.5 to 1962.5 and to reveal any changes in secular variation during this time. The epoch to epoch changes are much smoother than have been previously found, and permit an examination of the movements of the centred and eccentric dipoles and changes in westward drift. The secular variation has also been used to determine surface motions of the Earth's core. Changes in secular variation are presented in the form of charts of secular acceleration.